1. Field of the Invention
The present invention relates to clutchless variable displacement compressors. More particularly, the present invention pertains to controlling the displacement of a compressor by supplying the pressure in a discharge pressure zone to a pressure control chamber through a pressurizing passage while releasing the pressure in the control chamber into a suction pressure zone through a pressure releasing passage.
2. Description of the Related Art
Compressors are typically provided in vehicles to air-condition passenger compartments. Compressors capable of varying their displacement are preferred since they accurately control the temperature inside the passenger compartment and thus allow the environment in the compartment to be maintained at a comfortable level. Such a compressor, that is, a variable displacement compressor, typically has a tiltable swash plate, which is mounted on a shaft. The inclination of the swash plate is controlled based on the difference between the pressure in a crank chamber and the suction pressure. The rotating movement of the swash plate is converted to reciprocating linear movement of pistons.
U.S. Pat. No. 5,173,032, which corresponds to Japanese Unexamined Patent Publication No. 3-37378, describes a piston type compressor that does not employ an electromagnetic clutch. Generally, an electromagnetic clutch connects the compressor's drive shaft to an external drive source for transmission of driving power and disconnects the shaft from the drive source to stop transmission of the power. However, the external drive source and the drive shaft are directly connected to each other in the described compressor.
The elimination of the clutch and direct connection of the drive source with the drive shaft solves the problems of shocks, which would occur when connecting and disconnecting the clutch. By employing such compressors in vehicles, it is possible to provide further comfort to the driver and the passengers when driving the vehicle. Elimination of the clutch reduces the weight of the cooling apparatus and the costs of the compressor.
A typical clutchless compressor is operated even when cooling is unnecessary. When cooling is unnecessary, the displacement of the compressor should be minimized and formation of frost on the evaporator should be prevented. Circulation of refrigerant gas between an external refrigerating circuit and the compressor is stopped when cooling becomes unnecessary or when there is a possibility of formation of frost. The afore-mentioned U.S. Patent describes an electromagnetic valve that blocks the flow of gas from the external circuit to a suction chamber of the compressor and thus stops the circulation of gas between the external circuit and the compressor.
In this prior compressor, the pressure in the suction chamber decreases when the flow of gas from the external circuit to the suction chamber is stopped. This results in a displacement control valve, which detects the pressure in the suction chamber, being completely opened and thus permitting the gas in a discharge chamber to flow into the crank chamber and raise the pressure therein. The gas in the crank chamber is then supplied to the suction chamber. A circulating passage is thus defined extending between cylinder bores, the discharge chamber, the crank chamber, the suction chamber, and the cylinder bores.
The pressure decrease in the suction chamber also lowers the pressure in the cylinder bores. Thus, the difference between the pressure in the crank chamber and the pressure in the cylinder bores becomes large. This minimizes the inclination of the swash plate, which reciprocates the pistons, and results in the displacement becoming minimum. In this state, the drive torque required to operate the compressor becomes minimum and power loss, which occurs when cooling is unnecessary, is minimized.
By closing the electromagnetic valve, the flow of gas from the external refrigerating circuit to the suction chamber is brought to a stop. The electromagnetic valve is attached to an inlet of the compressor, from which refrigerant is introduced. Therefore, since the electromagnetic valve is used together with the control valve, the structure of the compressor is complicated. This results in high costs.